Alimilnum-magnesiijm casting alloys



States Unite ALUMINUM-MAGNESIUM CASTWG ALLOYS No Drawing. ApplicationDecember 12, 1950, Serial No. 200,496

2 Claims. (Cl. 75-447) This invention relates to aluminum alloys inwhich magnesium is a major alloying element and it relates moreparticularly to new and improved aluminum-magnesium alloys for use incasting.

Form a commercial standpoint, aluminum-magnesium casting alloys may becatalogued into two distinct groups. Casting alloys having a magnesiumcontent ranging from 9 to 12 percent by weight are responsive to heattreatment for the development of improved physical properties. By heattreatment, the aluminum-magnesium intermetallic compounds are put intosolid solution from which they are reprecipitated under room conditionsin finely divided form as compared to the coarse crystals in which formthey existed in the original casting. The major portion of thereprecipitation occurs Within the first few days of aging so thatimprovement in physical properties are rather quickly developed.

In the group containing 1 to 9 percent magnesium, heat treatment hasvery little efiect on the physical properties of the casting. Alloyswithin this lower range of magnesium content form the subject matter ofthis invention. The physical properties which are developed by the alloyfrom casting are generally referred to as the as cast properties. In thepast, it has been possible further to subdivide this group containinglower magnesium content with respect to the method of casting; that is,the group may be divided into alloys suitable for casting in green sand,hereinafter referred to as sand casting, and it'may be divided intoalloys suitable for casting into permanent molds, hereinafter referredto as chill casting. The latter casting technique may relay entirely ongravitational force or positive pressures may be employed for fillingthe molds with the melt, as in die casting. The chief dillerence betweensand casting and chill casting resides in the rate of heat transferthrough the mold walls, it being greater in chill casting with theresult that crystal formation and solidification occurs at a more rapidrate.

Chill casting usually has the elfect of producing castings havingsmaller grain size. In aluminum-magnesium alloys, components, such asmagnesium, present in quantities above their normal solid solubilitylimit at room temperature are retained in a metastable condition ofsolid solution instead of precipitating out as in the slower coolinginherent in sand casting methods. Ordinarily, these characteristics in ametal or alloy lead to improved physical properties, but the reverseeffects are obtained with' aluminum-magnesium alloys. 'No satisfactoryexplanation has been advanced for this opposed reaction in the behaviorof aluminum-magnesium alloys whereby they fail to develop superiorproperties responsive to finer grain size and increased amounts ofmagnesium in solid solution. No one, to my knowledge, has been able tomanufacture an aluminum-magnesium alloy adapted for chill casting whichhas physical properties that are as high or higher than those obtainedby casting the same alloy in green sand.

It is an object of this invention to produce an alumi- 2,733,991Fatented Feb. 7, 1956 hum-magnesium alloy which is not subject to thelimitations and does not exhibit the undesirable expected reactions ofprior art composition.

Another object is to produce an aluminum-magnesium alloy which can beused interchangeably for chill and sand casting without sacrificing anyphysical properties.

A further object is to produce an aluminum-magnesium casting alloy whichhas improved physical properties substantially equivalent to thosesecured by heat treatment of other types of heat treatable alloys.

A still further object is to produce an aluminum-magnesium casting alloythat has properties far superior to alloys heretofore secured by eithersand casting or by heat treatment; that has excellent tensile strengthand ice ductility without the necessity of heat treatment; that ishighly resistant to corrosion and retains high luster; that hasexcellent machine properties; that acquires and retains a brilliantsurface which can be developed by simply polishing; that develops highmechanical strength immediately upon cooling to room temperature, whichstrength properties are not subject to change with age as compared tothe heat treated castings which ultimately become embrittlecl.

A still further object is to produce an aluminum alloy containing 1 to 9percent magnesium as a major alloying element and small amounts oftitanium, beryllium and manganese as minor alloying elements to providefor specific improvements in physical characteristics of the cast alloywhereby excellent combinations of tensile strength, yield strength andelongation are developed without the necessity of employing expensiveheat treatment.

In accordance with this invention, a new and improved casting alloywhich is capable of markedly improved physical properties in as castcondition is composed primarily of magnesium as the major alloyingelement with aluminum and titanium, beryllium and. manganese as minoralloying elements.

As previously pointed out, invention herein is directed toaluminum-magnesium type alloys in which the range of magnesium islimited to between 1 to 9 percent by weight and particularly to between3 to 9 percent by weight. Optimum results are secured when the magnesiumcontent in the alloy is within the range of 6.0 to 8.5 percent byweight. In the past, the best aluminum base alloys having 6.0 to 8.5 percent by weight magnesium were characterized by a tensile strength ofabout 32,000 pounds per square inch and elongation of about 10 per centper inch.

in the practice of this invention, I have been able to produce an alloywhich averages a tensile strength in excess of 42,000 pounds per squareinch and more than 16 per cent elongation. The properties secured bythis practice is the more important because it provides alloys havingphysical properties in the as cast condition which are comparable toheat treatable alloys having 9 to 12 per cent magnesium.

Titanium in amounts ranging from 0.01 to 0.40 percent by weight may beused, but it is best to maintain the titanium content to within therange of 0.10 to 0.25 percent by Weight. When more than 0.25 percent byweight titanium is employed, complete solution is difficult to maintainand precipitation of titanium occurs as the alloy passes through itsfreezing range. The metallic and the intermetallic compounds that areformed have a tendency to make the melt more sluggish and excesses oftitanium are found to be undesirable when the alloy is used for sometypes of casting. However, as much as 0.40 percent titanium may be used.

The beryllium content varies in accordance with the type of casting. Forsand casting, it is best to hold the beryllium content within the rangeof 0.0005 to 0.03 percent by weight and for chill casting the berylliumcontent is best if maintained within the range of 0.0005 to 0.20 percent by weight. The additional benefit secured by the use of berylliumin amounts in excess of 0.07 percent does not warrant the additionalcost. For chill casting, best results are secured when the amount ofberyllium is above 0.001 per cent by weight.

I have found that metallurgical data with respect to the effects ofmetal additions to aluminum alloys for the purpose of improving strengthand hardness does not usually hold true for aluminum-magnesium alloysand particularly for aluminum-magnesium alloys in which titanium andberyllium constitute alloying elements. For example, it has beensuggested that copper, iron, silicon, zinc or zirconium have theproperties of increasing hardness and strength of aluminum metal and itsalloys, yet I have found that these same metals, when used in quantitieswhich might be expected to improve yield strength, are highlydetrimental when the alloy is composed of aluminum, magnesium,beryllium, and titanium.

Of the metals alloyed with aluminum, magnesium, beryllium, and titaniuml have found that manganese has a very desirable effect in increasingyield strength without reducing tensile strength or elongation which isthe usual reaction in most alloys.

In the practice of this invention, the desirable results are securedwhen manganese is present in the alloy in amounts ranging from 0.001 to1.2 percent by weight. For sand casting best results are secured whenmanganese is present within the range of 0.05 to 0.30 percent by weightwhile for chill casting the most suitable range is within 0.20 to 0.60percent by weight.

The following is an example of a sand cast alloy formulated ofcomponents without manganese as compared to the physical properties ofan alloy embodying the same elements prepared under substantially thesame conditions with the addition of manganese in accordance with thepractice of this invention:

By comparison, it will be evident that the addition of manganese as aminor alloying element in combination with titanium and beryllium andthe major alloying element of magnesium with aluminum provides for avast improvement in yield strength. It will be evident that a new highin strength properties is secured in an aluminummagnesium casting alloy,properties which greatly exceed values considered possible for suchalloys in as cast" condition. As a result, many new and very importantuses have been found for such material.

One of the most important advancements made by this alloy resides in thedevelopment of a new group of aluminunnmagncsium casting alloys whichcan be used interchangeably with substantially equal success for bothgreen sand casting and for chill and die casting. This is unusual inaluminum-magnesium alloys because of the vast differences that exist intheir rate of crystallization and freezing whereby finer grain size andretention of greater proportions of metals in solid solutions arecharacteristic of chill casting. Before, it was necessary to formulateseparately for each type of casting with the result that it wasdifiicult for one industry to engage in various casting techniques.

The following is an example showing improvement in all physicalproperties in a chill cast alloy embodying features of this invention:

Boron may be included to advantage as a minor alloying element in thepreparation of alloys embodying features of this invention. Whenpresent, the amount of boron should not exceed 0.01 percent by weightbecause greater amounts seem to precipitate out as intermetalliccompounds. Although greater proportions of boron may be used, theproperties of the alloy are sometimes harmfully affected.

Alloys used in chill casting, especially when formed in molds heated to600 to 900 B, have been found better in many respects than those securedby the most favorable alloys in sand casting. In production, the alloymay be compounded by the addition of metallic components to moltenaluminum maintained at least above melting temperature. To the moltenaluminum, the other elements may be added in desired amounts in a formwhich is relatively free of harmful impurities. With some of theelements it is best to alloy with pure metals or master alloys ofaluminum, while with other, additions may best be made through theaddition of reducible inorganic salts such as halides from which themetal is made available and from which gases are released for sweepingimpurities out of the molten alloy. For example, beryllium can beincorporated to best advantage as a master alloy with aluminum whiletitanium is added to greatest advantage as the inorganic salts. Whenadded in this manner greater amounts of titanium can be incorporatedsince titanium so produced remains in solution.

For best results the total impurities which includes metals of the typecopper, iron and silicon should be held below 0.45 percent by weightwith 0.25 percent by weight being the maximum for any one of theaforementioned metals. Since alkali metals, especially sodium, arehighly deleterious to the physical properties of the alloy, inclusion ofmore than 0.001 percent by weight thereof should be avoided.

It will be understood that numerous changes may be made in the amountsof materials, their methods of incorporation in the alloy and itsfabrication into a cast product without departing from the spirit of theinvention especially as defined in the following claims.

I claim:

1. Au aluminum base casting alloy consisting of 3 to 9 percent by weightmagnesium, 0.0005 to 0.07-percent by weight beryllium, 0.1 to 0.25percent by weight titanium and 0.05 to 0.60 percent by weight manganese,the balance being aluminum, said alloy containing not over 0.45 percentby weight impurities including a maximum of 0.25 percent by weight of anelement selected from the group consisting of copper, silicon and ironand not over 0.001 percent by weight of an alkali metal.

2. An aluminum base alloy for sand casting consisting of 3 to 9 percentby weight magnesium, 0.0005 to 0.03 per cent by weight beryllium, 0.10to 0.25 percent by weight titanium, 0.05 to 0.30 percent by weightmanganese, the balance being aluminum said alloy containing a maximum of0.45 percent by weight impurities including a maximum of 0.25 percent byweight ofan element selected from the group consisting of copper,silicon'and iron.

(References on following page) References Cited in the file of thispatent UNITED STATES PATENTS Beck May 11, 1937 Stroup Dec. 14, 1943Willmore Aug. 14, 1951 Cooper Jan. 22, 1952 FOREIGN PATENTS SwitzerlandAug. 1, 1932 Great Britain Apr. 26, 1950 OTHER REFERENCES Foundry TradeJournal, Nov. 17, 1938, pages 373

1. AN ALUMINUM BASE CASTING ALLOY CONSISTING OF 3 TO 9 PERCENT BY WEIGHTMAGNESIUM, 0.0005 TO 0.07 PERCENT BY WEIGHT BERYLLIUM, 0.1 TO 0.25PERCENT BY WEIGHT TITANIUM AND 0.05 TO 0.60 PERCENT BY WEIGHT MANGANESEANCE BEING ALUMINUM, SAID ALLOY CONTAINING NOT OVER 0.45 PERCENT BYWEIGHT IMPURITIES INCLUDING A MAXIMUM OF 0.25 PERCENT BY WEIGHT OF ANELEMENT SELECTED FROM THE GROUP CONSISTING OF COPPER, SILICON AND IRONAND NOT OVER 0.001 PERCENT BY WEIGHT OF AN ALKALI METAL.